The present invention relates to information recording disks such as an optical disk, and more particularly to an optical disk according to a novel method of dividing a recording region into parts and of forming a header-signal generating pre-pit train.
An optical disk of the modified constant angular velocity (MCAV) type has been proposed which is higher in recording density than an optical disk of the constant angular velocity (CAV) type.
In the optical disk of the MCAV type, a ring-shaped recording region is divided in radial directions into a plurality of concentric, annular blocks, each of the blocks being divided in a circumferential direction into a plurality of equal sectors so that the number of sectors included in the outermost block is about twice as large as the number of sectors included in the innermost block and sectors included in all the blocks have substantially the same length, and a pit train for generating a header signal is previously formed in each sector at a fixed pit pitch.
When this optical disk is rotated at a constant angular velocity, and an appropriate clock frequency is used for each block, a linear recording density can be made constant in the whole of the recording region extending from the inner circumference thereof to the outer circumference (refer to Japanese Patent Application JP-A-62-80,865).
A reproduced signal read out from an optical disk is separated into a data pulse and a clock pulse by a window-signal which is produced by a data separator, and then data is reconstructed.
The reproduced signal obtained from the optical disk includes jitter due to variations in rotational speed of a disk driving motor, variations in laser output power, variations in output of a variable frequency generator, variations in recording sensitivity on the optical disk, the deformation of the disk in a rotating state, and other reasons. In other words, a peak position in the reproduced signal is shifted before and behind a reference position in a writing operation. Accordingly, when the window width is not made large, the time margin of the peak shift for the window width, that is, a jitter margin is reduced, and thus a reproducing error is readily generated.
When the optical disk is rotated at a constant angular velocity, the jitter due to the above-mentioned factors is kept substantially constant at the inner and outer parts of the optical disk. In an optical disk of the MCAV type, however, a pit train for generating a header signal is previously formed at a constant pit pitch at the inner and outer parts of the disk, and thus the width of the window signal produced by the data separator is small at an outer block having a large linear velocity for the light spot. Hence, as the light spot is nearer to the outermost block, the jitter margin is reduced, and the reproducing error is easy to generate.
It is an object of the present invention to provide an information recording disk which can solve the above-mentioned problem of the prior, and is high in recording density and low in error rate.
In order to attain the above object, according to the present invention, there is provided an information recording disk having a structure that a ring-shaped recording region is divided into a plurality of concentric, annular blocks, each of the blocks is divided is  in a circumferential directions  direction into a plurality of equal sectors, and a recording track and pit train for generating a header signal are previously formed in the recording region so that the recording density of a block increases as the block is nearer to the outer circumference of the recording region, in comparison with constant angular velocity type, in which disk the recording region has a portion where blocks are pre-formatted so that the pit pitch of the header-signal generating pit train formed on the innermost recording track of the outer one of two adjacent blocks is larger than the pit pitch of the header-signal generating pit train formed on the innermost recording track of the inner block.
In a case where the characteristics of the variable frequency generator included in the data separator are kept constant, the width of the window signal produced by the data separator is increased as the pit pitch of the header-signal generating pit train is larger, and the time duration of the window signal is decreased as the linear velocity of recording track is larger.
According, when the pit pitch of the header-signal generating pit train is made large at outer blocks where a recording track has a large linear velocity, reduction in window width due to the large linear velocity of recording track is substantially cancelled by an increase in pit pitch of the header-signal generating pit train.
Although the window width is decreased by employing the large linear velocity, the decrease is minimized by intentionally increasing the pit pitch.